The present disclosure contemplates instrumentation and procedures for achieving spinal fixation or more particularly for percutaneously introducing a spinal fixation system into a patient.
A typical spinal fixation system 10 as shown in FIG. 1 spans between successive vertebrae V of the spine. An elongated member, such as rod 12, extends along the length of the spine and provides an anchor point for connecting each vertebra to the rod. The rod is typically contoured to approximate the normal curvature of the spine for the particular instrumented spinal segments, which may include lordosis or kyphosis. Anchor devices 15 are provided for connecting the vertebral segments to the elongated member. These anchor devices may include hooks, bolts, screws or other means for engaging a vertebra. For the purposes of the present discussion, the anchor device 15 is a bone screw assembly, such as the screw assembly shown in FIG. 2. However, it should be appreciated that the instrumentation and procedures disclosed herein may be implemented with other types of anchor devices, such as a hook engaged to the lamina of a vertebra for instance.
The bone engaging fastener or screw assembly 15 includes a shank 16 that carries threads configured to engage vertebral bone. For instance, the fastener is a pedicle screw with a shank that is threaded for engagement within the pedicle of the vertebra. The screw assembly further includes a head 16a by which the screw, and ultimately the vertebra, is fastened to the spinal rod 12. In particular, the head 16a supports a yoke 17 that is generally U-shaped to receive the spinal rod therethrough, as depicted in FIG. 2. The rod 12 may be supported in part by a collar 18 mounted over the head 16a of the bone screw. A cap 19 carries a set screw 20 that locks the rod within the yoke 17 and thus fastens the rod 12 to the bone screw.
One embodiment of a bone screw assembly 15 is disclosed in co-pending U.S. application Ser. No. 11/762,898 (the '898 application), entitled “Multi-Axial Fixation Assembly”, field on Jun. 14, 2007 and published as No. 2008/0119858, the disclosure of which is incorporated herein by reference. For the purposes of the present disclosure, the bone screw 15 may be constructed as disclosed in the '898 application, although it is understood that other bone screw or multi-axial fastener configurations may be implanted using the instruments and procedures disclosed herein. In the multi-axial bone screw assembly 15 the yoke 17 is articulatingly attached to the threaded bone screw 16, and more specifically to the head 16a of the bone screw, so that the yoke 17 can adopt a range of spherical angles relative to the bone screw. Thus, the yoke can articulate relative to the bone screw fastened in the vertebra so that the slot 42 can be aligned to receive the connecting rod 25.
While in the past spinal fixation systems have been implanted in open procedures involving relatively large incisions through the patient's tissue with significant muscle retraction, more recent procedures have been developed to percutaneously introduce spinal fixation systems in a minimally invasive manner. One technique known as the Sextant® System is described in U.S. Pat. No. 6,530,929, issued to Justis, et al. In the '929 patent, separate incisions are made for introducing respective pedicle screws each attached to a tubular extension extending outwardly from the patient through each incision. A pivot arm coupled to the extensions introduces an elongate rod through another separate incision remote from the incisions receiving the extensions. The pivot arm urges the rod beneath the skin and into the pedicle screws for fixation. Other percutaneous systems such as that shown in U.S. Pat. No. 7,306,603 issued to Boehm, Jr. et al. utilize tubular pedicle screw extensions to place a rod longitudinally through the extension into one of the pedicle screws. The rod is then pivoted about the pedicle screw through an incision between the pedicle screws to the second pedicle screw. Others still employ systems such as that shown in U.S. Pat. No. 7,250,052 issued to Landry et al. wherein slots in the screw extensions are used to guide a rod between the extensions through a single incision into position in two or more pedicle screws.
Nevertheless, there is current desire for minimally invasive instruments and procedures for the percutaneous placement of spinal fixation systems that are relatively simple and easy to use and that provide for enhanced assurance of rod introduction and connection to the spinal implants.